This invention relates to tunable acoustic-optical filters and is more particularly concerned with electronically tuning acoustic-optical filters.
Tunable acousto-optical filters (TAOF) utilize the interaction of light and acoustic beams in an optically anisotropic medium. A narrow band of light is selected to be passed or rejected. The center wavelength of the band is a function of the acoustic wave frequency.
The optically anisotropic medium allows propagation of both optical and acoustic beams. The medium is usually a birefringent crystal having a particular orientation with respect to the vectors of the optical and acoustic beams.
The acoustic beam is launched into the medium by a transducer or coupling means and travels through the medium as waves. As a peak of the acoustic wave passes through the medium, it locally perturbs or distorts the small scale geometry of the medium, causing changes in its optical properties. One band of optical frequencies will be most strongly affected by the changes in the medium induced by the acoustic wave. The affected band may be distinguished from the remainder of the optical spectrum by additional elements of the filter, such as polarizers, which may be arranged to either pass or reject the band while having the opposite effect on other frequencies.
TAOF's are classified as being collinear or non-collinear. The acoustic and the optical waves each has group velocity vectors and phase velocity vectors which are not necessarily aligned. In a collinear filter, the phase velocity vector of the acoustic-wave is collinear with the optical wave propagation.
A detailed description of a collinear TAOF is given in U.S. Pat. No. 3,679,288 granted to S. E. Harris. Light to be filtered is first linearly polarized before entering the medium as an extraordinary wave. A transducer induces an acoustic wave through the medium. Light at a frequency related to the acoustic frequency is diffracted into a polarization orthogonal to the polarization at the input. A polarization analyzer discriminates between the affected frequencies and other frequencies.
The relations between the center of the affected optical band and the acoustic frequency is given by Harris as: ##EQU1## where: f.sub.a is the acoustic frequency
.lambda..sub.o is the wavelength of the center of the band, PA0 .DELTA..eta. is the birefringence of the material, and PA0 V.sub.a is the acoustic velocity in the medium
I. C. Chang in U.S. Pat. No. 4,052,121 describes a non-collinear filter having the approximate relation ##EQU2## I. C. Chang, at pages 370-372, Applied Physics Letters, Vol. 25, No. 7, Oct. 1, 1974, gave as a general relationship: ##EQU3## Saito et al in U.S. Pat. No. 3,944,335 gave a relationship which yields: ##EQU4## In these last three relationships .theta..sub.i is the angle of the light beam with respect to an axis of the medium.
If .theta..sub.i is constant, these relations reduce to: ##EQU5## C is a constant ##EQU6## C is a constant
Three tuning curves are plotted in FIG. 1 and were originally based on the above relationships.
One curve is from Harris U.S. Pat. No. 3,679,288 for a collinear filter having LiNbO.sub.3 as the anistropic medium.
The second curve is for a collinear CaMoO.sub.4 filter from the work of S. E. Harris and S. T. K. Nieh, pages 223-225 of Vol. 17, No. 5 of Applied Physics Letters Sept. 1, 1970.
The third curve, from I. C. Chang, U.S. Pat. No. 4,052,121, is for a non-collinear TeO.sub.2 filter.
The scale of the acoustic wave frequency is different for each curve, so as to better demonstrate the similarity of the curves when normalized. It will also be observed that the acoustic frequency may be at radio frequencies, and is not to be confused with audible sounds.
In addition to the theoretical relations previously stated, a known empirical relationship which approximately fits actual tuning curves is: EQU f.sub.a =A/.lambda..sub.o -B
wherein A and B are determined by the acoustic velocity in the medium, V.sub.a, and the birefringence of the medium, .DELTA..eta.. Both parameters, particularly birefringence, change with temperature. Since V.sub.a and .DELTA..eta. are temperature dependent, the tuning curve tends to shift and change shape as the medium heats. Thus, a tuning curve obtained at a given temperature will give inaccurate results at other temperatures.
Temperature changes in the TAOF medium are to be expected. Acoustic transducer inefficiencies, conversion of acoustic energy to heat by losses in the medium and absorption by an acoustic load, as well as the effect of ambient temperature, contribute to heating the medium. It is difficult to directly measure the effective medium temperature. There also may be a thermal gradient across the medium.
One object of the invention is to temperature compensate a TAOF.
An additional object is to obtain a tuning relationship corrected for the effective temperature of the medium of a TAOF.
Another object is to determine the acoustic frequency corresponding to the center wavelength of an optical band at a reference condition and to utilize said acoustic frequency to provide a temperature corrected tuning curve.